(a) Field of the Invention
This invention relates to a process for the purification of 3,3'-dinitrodiphenyl compounds such as 3,3'-dinitrodiphenylsulfone (hereinafter abbreviated as "3,3'-DNDS") and 3,3'-dinitrobenzophenone (hereinafter abbreviated as "3,3'-DNBP").
(b) Description of the Prior Art
3,3'-DNDS and 3,3'-DNBP are converted respectively into their corresponding 3,3'-diamino compounds, namely, 3,3'-diaminodiphenylsulfone and 3,3'-diaminobenzophenone, which are useful as heat-resistant high molecular monomers as well as intermediates for agricultural chemicals, medicines and dyes. These compounds are important especially as raw materials for heat-resistant polyamides, polyamide-imides and polyimides and as hardeners for epoxy resins.
It has conventionally been known to prepare 3,3'-DNDS by nitrating diphenyl sulfone with a mixed acid [J. Org. Chem., 4 262 (1939); Chem. Abs., 19 980 (1925)] or by subjecting diphenyl sulfide to simultaneous oxidation and nitration with fuming nitric acid [Chem. Abs., 40 7153 (1946)]. It has also been proposed to isolate 3,3'-DNDS as a byproduct upon nitration of benzene in the presence of sulfuric anhydride [Chem. Abs., 73 34962n (1970)]. It has also been known to isolate 3,3'-DNDS as a byproduct upon preparation of m-nitrobenzenesulfonic acid through sulfonation of nitrobenzene [Chem. Abs., 73 34962n (1970)].
It is however necessary to conduct recrystallization repeatedly by using a great deal of solvent for the isolation of 3,3'-DNDS in its purified form in the preparation process of 3,3'-DNDS from diphenyl sulfone or diphenyl sulfide, because the reaction product obtained by the nitration is a mixture of isomers which have been substituted by nitro groups at different positions. The yield is therefore lowered and substantial cost and labor are required for the recovery of the solvent used in the purification, the treatment and disposal of residual matter, etc., resulting in an increase to its price. As mentioned above, 3,3'-DNDS is also obtained as a byproduct upon nitration of benzene in sulfuric anhydride or sulfonation of nitrobenzene. This approach is however accompanied by such drawbacks that its yield is low and since it is not prepared as the principal reaction product, its supply is not satisfactory and cannot meet the increased demand for same.
On the other hand, 3,3'-DNBP can be prepared by the nitration of benzophenone. In this process, the reaction product is a mixture containing isomers. In order to isolate the intended product, namely, the 3,3'-dinitro isomers only, it is indispensable to conduct its purification, i.e., recrystallization repeatedly by using a great deal of solvent [Journal of Chemical Society, 125 767 (1924)]. For this reason, the yield of 3,3'-DNBP is lowered to a significant extent and moreover, cumbersome steps and additional expenses are also required for the recovery of the solvent employed in the purification, the treatment and disposal of residual matter, etc.
It has recently been proposed to conduct the nitration of benzophenone in a large volume of fuming sulfuric acid so as to improve the selectivity for the meta-position [Journal of Organic Chemistry, 46 5014 (1981)]. As drawbacks of this process, may be mentioned that it requires fuming sulfuric acid, the concentration of which must be controlled within a narrow range, in a large volume and moreover, stringent control of reaction temperature and time is essential. In addition, the above process is accompanied by potential danger because the temperature has to be raised to 70.degree. C. in spite of the use of fuming sulfuric acid of a relatively high concentration. Even when the process is carried out under such conditions, the purity of the resultant 3,3'-dinitro isomer is as low as 93.5%-93.7%.
Describing the above-proposed process further in detail, its characteristic feature resides in that benzophenone is dinitrated with a mixed acid, which contains fuming sulfuric acid at a high concentration in order to increase the selectivity for the 3,3'-dinitro isomer, and the reaction mixture is then heated to sulfonate the 2,3'-dinitro isomer and the like, byproducts of the nitration reaction, in a short period of time so as to facilitate their removal in subsequent treatment and disposal steps. The content of the 3,3'-dinitro isomer is 81-82% upon completion of the nitration. Since the reaction mixture is thereafter subjected to a heat treatment, mononitro derivatives, 2,3-dinitro isomer and the like are eliminated and depending on the conditions of the heat treatment, the amount of the 3,3'-dinitro isomer is also reduced. As a result, the yield of the 3,3'-dinitro isomer drops to 60-84%.
As has been described above, it is necessary to solve problems on the materials of apparatus for the above-described processes and to make a great deal of waste acids of high concentrations pollution-free. Moreover, the reaction products are mixtures of isomers and their purification is hence indispensable. Accordingly, the above-described processes are accompanied by various difficulties for their practice on industrial scales.